Studies on utilization of fuel cells and solar cells as a power supply for mobile devices have been in progress. The reason being that the fuel cells have high energy density per unit weight and hence a large capacity, moreover, the solar cells are portable due to their light weight and thin structure.
The fuel cells produce power from a chemical reaction between hydrogen and oxygen. The fuel cells are considered to produce clean energy because they neither discharge noxious gases such as nitrogen oxides (NOx) nor make any noise. The weight energy density of the fuel cells, one of the indicators that meter the performance of the cells, is said to be ten times as high as that of lithium ion cells. This means that a 5-hour-driving note-type personal computer can be used for 50 hours by employing the fuel cells. Due to these advantageous, the fuel cells are expected to drastically enhance convenience of the mobile devices.
The solar cells are clean energy sources free of noxious gases and noise and have an advantage over secondary cells, such as lithium ion cells and nickel-cadmium cells, that energy does not have to be supplemented. Therefore, the solar cells alone or in combination with fuel cells are expected to be used more and more in mobile devices.
Solar cells having a size suitable for use in the mobile devices have a low output voltage of about 0.5 volt (V) for a single cell. A solid polymer electrolyte fuel cell (PEFC) and a direct methanol fuel cell (DMFC), which are expected to be used in the mobile devices, each has a low output voltage for a single cell of 0.6 V to 0.7 V without loads and around 0.3 V when rated output is produced. These output voltages are determined based on the principles of power generation by fuel cells and solar cells. Single cells alone cannot produce output voltages higher than the above-mentioned voltages.
Therefore, with single cell batteries alone, neither the electronic devices can be operated nor the secondary batteries, such as a nickel-cadmium battery and a lithium battery, can be charged. This compels one to use, for example, a technique of serially connecting single cells to form a battery module to obtain a voltage necessary for operating the electric/electronic devices or charging the secondary cells.
However, this technique has the following problems.
First, in the case of fuel cells, there is an increase in cost for producing the fuel cells due to the structure for uniformly distributing fuel and oxygen (air) to all the cells. Moreover, an output current produced by the structure is limited to a current from a cell that receives a minimum supply of fuel and oxygen or to a current of a cell that generates the least current due to an inappropriate mixing ratio of the fuel and oxygen. Therefore, a countermeasure is taken for uniformly distributing fuel and so on, such as providing grooves in flow channels for fuel and oxygen in the fuel cell. However, this increases the cost since the grooves need to be provided with a coat of a material that can endure corrosion.
On the other hand, two problems arise in case of the fuel cells. The first problem relates to the output power. Some of the single cells that constitute a solar cell module may be under shadow, which leads to a decrease in the output voltage of the solar cells. In particular, when the solar cell module is mounted on a mobile device, it can be difficult for the entire solar cell module to receive light always. In addition, if a structure is adopted in which the entire solar cell module receives light always, it may not suit to the user's satisfaction.
The second problem relates to the cost. To connect single cells of a solar cell serially to constitute a solar cell module, it is essential to add bypass diodes, and to take countermeasures for insulation of wiring that connects a front surface of one solar cell to a rear surface of a neighboring solar cell and between the single cells of the solar cell. To increase the module packing factor of the solar cell, it is necessary to shorten the wiring between the single cells of the solar cell or decrease interstices for intercellular insulation. This requires the cells to be arranged with high precision. The countermeasures for intercellular insulation and the requirement of high precision are some of the reason that increase the cost.
Conventional technologies aimed at solving the problems include solar cell devices that use a tandem-type solar cell providing a relatively high output voltage of a little less than 2 V to avoid serial connection and use a booster circuit to charge a secondary cell (see, for example, Patent Document 1).
Patent Document 1: Gazette of Japanese Patent No. 3,025,106 (for example, page 3, reference numeral 5 in FIG. 1)
In the solar cell device described in Patent Document 1, a tandem-type solar cell is used in which the output voltage of the solar cell is boosted by constructing the solar cell by a plurality of layers and connecting the layers directly one to another. Since the tandem-type solar cell provides an output voltage of a little less than 2 V, the solar cell can boot a booster circuit with an oscillation circuit of a complementary metal oxide semiconductor (CMOS) type having a minimum start-up voltage of about 1.4 V.
The tandem-type solar cells are cheaper only in comparison with a solar cell in which single cells are serially connected. In other words, when compared with ordinary single cell solar cells, there still remains a problem of complexity in the production process, so that the production cost cannot be reduced drastically and the cost of the solar cell is not reduced.
The solar cell device disclosed in Patent Document 1 is provided with a booster circuit and it is necessary to first boot the booster circuit before the solar cell device can be operated. A predetermined amount of start-up energy needs to be supplied from a power supply unit. Accordingly, when the energy in the power supply unit is empty or insufficient, the booster circuit cannot be boot.
A first object of the present invention is to provide a booster that prevents an increase in production cost due to use of a special electric cell and allows a reduction in cost by multipurpose application of cells.
A second object of the present invention is to provide a booster having a booster circuit that can be boot without a power supply unit.